A sick boy lies on his mother's lap inside a hospital after he consumed one of the contaminated meals. Photo: Reuters

The principal, Meena Kumari, and her husband, Arjun Rai, went into hiding almost as soon as the young children in Ms Kumari's care fell ill at the school in the village of Gandaman in the Saran district of Bihar, a poor state in India's north-east.

Police arrested Ms Kumari a week after the deaths, while Mr Rai absconded for nearly two months. Murder charges were filed against the couple on Sunday night.

Investigators have determined that Ms Kumari provided the school's cook with cooking oil laced with the same kind of insecticide that Mr Rai sold to local farmers.

Of the 52 children attending school on July 16, 47 ate the poisoned food, although many objected to the foul smell. Ms Kumari scolded the complaining children and insisted they eat the meal of rice, soybeans and potato curry.

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All 47 children became seriously ill and 23 died, according to Raj Kaushal, a police officer who investigated the case. The children were six to 12 years old.

India's midday meal program is one of the most important anti-poverty programs in the world and is a vital source of nutrition for India's children, nearly half of whom suffer some form of malnourishment.

The program has also helped raise school attendance in India, where literacy rates remain woefully low.

But food quality and safety standards in the program have long been poor, and the deaths in Bihar have led to a spate of stories about less serious food poisoning problems at other schools.

P.K. Shahi, Bihar's minister of human resource development, said in an interview that the state had fixed some of the problems in its school lunch programs by training cooks and building new kitchens, but he noted that the state pays school cooks a salary of just 1000 rupees (about $17) a month.

''What kind of quality can be expected with so little money?'' Mr Shahi said.

The Gandaman school remains closed more than three months after the deaths, since many local parents refuse to allow their children to attend.

Satender Ram, 29, said that he lost his 10-year-old son, Rahul.

''Rahul was my only son, and I wanted to make him an engineer,'' Mr Ram said. ''With his death all my hopes are gone.''

Mr Ram has two daughters, both younger than Rahul, but they have been out of school for months because his wife fears for their safety.

''She does not want our daughters to meet the same fate,'' Mr Ram said.